As is known, the request for microsensors of small dimensions has led to the study of integrated solutions that apply the techniques and know-how acquired in the sector of the manufacture of semiconductors. In particular, integrated sensors for detecting acoustic waves have been studied that use piezoelectric layers, wherein a piezoelectric material layer, arranged between two electrode layers, overlies a cavity and forms an acoustic resonator (see, for example, “Bulk Acoustic Wave Theory and Devices” Joel F. Rosenbaum Artech House Inc., 1988).
These electro-acoustic resonators have been proposed for producing sensors of different types, such as force sensors, pressure sensors, acceleration sensors, weight sensors, and sensors for detecting chemicals, which exploit the variation of the oscillating frequency of the acoustic resonator following a variation of its mass and/or of its geometrical configuration. In practice, the resonator forms an integrated microbalance based upon the piezoelectric effect.
Recently, the use of microbalances as chemical sensors (electronic noses) has awakened particular interest. These sensors find in fact application in the foodstuff sector, where they can be used for controlling the degree of freshness of foodstuffs in the fishery industry (fish, molluscs, etc.), for assessing the degree of seasoning of cheese, for controlling the suitability of packaging, for controlling cooking of foodstuffs, for assessing the quality of beer, liqueurs and spirits. Integrated chemical sensors can moreover be used also in the cosmetics and pharmaceutical industry for controlling perfumes and aromas. The sector of environmental monitoring and that of medicine represent, instead, emerging markets for electronic noses. In both of these last fields they can be used for detecting chemical species produced by bacteria, for example, in environmental applications, for detecting cyanobacteria present in lakes and rivers, or in the medical field for detecting the presence of Escherichia coli. Finally, a market that represents an outlet that is very promising from the economic standpoint for electronic noses or, more in general, for automatic gas-detection systems is represented by the automotive sector. In this field, manufacturers are interested in controlling the quality of the air in the passenger compartment of vehicles and in controlling the exhaust gases.
For application as an electronic nose, an apparatus has been proposed that comprises a plurality of quartz chemical sensors, each formed by a quartz region having a surface covered by an adsorbent layer, which is able to bind in a non-selective way with the volatile substances present in the environment (ITRM2001A000455). In practice, the quartz forms, with an associated oscillating circuit, an electrical resonator having a natural resonance frequency comprised between 1 MHz and 20-30 MHz. Each sensor is provided with a different adsorbent layer. When the chemical substances in the environment (analytes) are adsorbed by one or more chemical sensors, the latter increase in weight, varying their own masses, and thus their own oscillating frequency. Alternatively, the relaxation time of the oscillations is measured.
A processing electronics connected to the chemical sensors processes the generated signals and compares them with known configurations in order to recognize the chemicals.
Known quartz sensors may, however, undergo improvement, in particular as regards sensitivity and the overall dimensions, which do not enable use thereof in many applications. The use of quartz renders moreover production complex and burdensome.
Sensors on silicon substrates have moreover been proposed, having cavities obtained by “bulk micromachining” using tetramethyl-amonium hydroxide (TMAH) (see for example “Sensors and Microsystems: Proceedings of the 10th Italian Conference” A. G. Mignani, R. Falciai, C. Di Natale, A. D′Amico, World Scientific Publishing Company, July 2008). This solution envisages deposition, on a surface of a silicon wafer, of a silicon nitride layer, operating as an etch stop, a first aluminium layer (bottom electrode), an aluminium nitride layer (piezoelectric material), and a second aluminium layer (second electrode). Then, an anisotropic back etching is performed, which is stopped on the silicon nitride layer, and the wafer is diced. In each die thus obtained, the stack of layers on the front defines a diaphragm, whereon a thin layer of a sensitive material, such as porphyrin, is deposited.
In this process, the required type of machining is costly and the use of TMAH is not usual in present production lines for integrated circuits. In addition, the etching procedure causes the formation, in the substrate, of a cavity with a trapezium cross-section having a minor base formed by the diaphragm and sides inclined by 45°-50°. Given that the thickness of the substrate is generally 675-700 μm, the major base of the cavity occupies an area having a side or diameter of 1.2-1.4 mm added to the width of the diaphragm. The minimum total area required by each microbalance is thus much greater than that due to the oscillating region alone. The microbalance thus has large overall dimensions, which reduces the possibility of integration thereof.
Consequently, in general, the known sensors do not provide the desired sensitivity, involve complex manufacturing processes, present high costs and dimensions such as not to enable a wide application thereof.